In general, the back flange length of a mount in a television camera varies among individual cameras even though they satisfy the same standard. Therefore, after a lens is mounted in a television camera, the back focus is adjusted so that the image pick-up plane of the television camera coincides with the image plane of the lens. Back focus adjustment is performed by moving one or more lens elements of a lens group in order to adjust the image plane position.
A prior art zoom lens used in a broadcast television camera that employs a tricolor separation prism is shown in FIG. 17. This zoom lens includes an afocal part and a lens group having a complex structure that is moved for adjusting the axial position of the image plane. Therefore, aberrations are sufficiently corrected and lens performance is sufficiently maintained after the lens is moved in order to make the position of the image plane coincide with the image pick-up plane of the television camera.
As shown in FIG. 17, wherein X is the optical axis, the zoom lens of Prior Art Example 1 is formed of, in order from the object side, a first lens group I having a first lens L1 through a fifth lens L5, a second lens group II having a sixth lens L6 through a ninth lens L9, a third lens group III having a tenth lens L10 and an eleventh lens L11, and a fourth lens group IV having a front subgroup IVa and a rear subgroup IVb. The front subgroup IVa is formed of a twelfth lens L12 through a fifteenth lens L15 and the rear subgroup IVb is formed of a sixteenth lens L16 through a twenty-first lens L21. A diaphragm stop 1 is positioned on the object side of front subgroup IVa of the fourth lens group. A tricolor separation prism 4 and a filter 2 are positioned between the rear subgroup IVb of the fourth lens group and the image plane R42. An image sensor 3, in this instance a CCD, is provided at the image plane. The image sensor 3, tricolor separation prism 4, and filter 2 are provided in the television camera body.
Table 1 below shows the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index Ne and Abbe number xcexde (both at the e-line) of the lens elements of the zoom lens of Prior Art Example 1.
Table 2 below shows at both the wide-angle end (WIDE) and telephoto end (TELE) for Prior Art Example 1: the focal length f (in mm); the F-number FNO with the diaphragm stop fully open; and the on-axis spacings D10, D18 and D21 (in mm) between the lens groups. Also listed, for the wide-angle end (WIDE) of Prior Art Example 1 are: the back focus Bf (in mm) of the zoom lens; the back focus Bf4b of the rear subgroup of the fourth lens group that is moveable in order to adjust the back focus; the ratio Bf4b/Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R29) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R30) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/FN; and the amount of axial shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 1) 1 mm toward the image so as to adjust the back focus. In the bottom portion of the table are listed the image sensor diameter (hereinafter termed the xe2x80x9cscreen sizexe2x80x9d, in mm) for which the zoom lens of Prior Art Example 1 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open.
FIGS. 19(a) and 19(b) show the spherical aberration and the astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 1 with the image plane position as designed (i.e., as given in Table 1 above). FIGS. 19(c) and 19(d) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 1 after the rear subgroup IVb has been moved (from the position indicated in Table 1 above) 1 mm toward the image so as to adjust the back focus. In FIGS. 19(b) and 19(d) the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. As is apparent from FIGS. 19(a)-19(d), the zoom lens of Prior Art Example 1 does not show significant differences in spherical aberration and astigmatism after the rear subgroup IVb has been moved (from its position indicated in Table 1) 1 mm toward the image so as to adjust the back focus. In fact, the lens performance changes very little when the rear subgroup IVb is moved 1 mm toward the image from its design position.
Unlike the zoom lens of Prior Art Example 1 described above, a zoom lens used in a simple television camera such as a CCTV (hereinafter termed Prior Art Example 2) is formed of a simple and compact lens group that is moved for adjusting the back focus. Therefore, the lens performance will inevitably change in association with the back focus adjustment. The basic lens element configuration of the zoom lens of Prior Art Example 2 is shown in FIG. 18.
As is shown in FIG. 18, the zoom lens of Prior Art Example 2 is formed of, in order from the object side, a first lens group I having a first lens L1 through a third lens L3, a second lens group II having a fourth lens L4 through a sixth lens L6, a third lens group III having a seventh lens L7, and a fourth lens group IV having an eighth lens L8 through a fourteenth lens L14. The fourth lens group IV further is organized as a front subgroup IVa formed of an eighth lens L8 through an eleventh lens L11, and a rear subgroup IVb formed of a twelfth lens L12 through a fourteenth lens L14.
A diaphragm stop 1 is positioned between L8 and L9. A filter 2 is positioned between the rear subgroup IVb and an image sensor 3 such as a CCD. The image sensor 3 and filter 2 are provided in the television camera body. X is the optical axis in FIG. 18.
Table 3 below shows the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index Ne and Abbe number xcexde (both at the e-line) of the lens elements of the zoom lens of Prior Art Example 2.
Table 4 below shows at both the wide-angle end (WIDE) and telephoto end (TELE) for Prior Art Example 2: the focal length f (in mm); the F-number FNO with the diaphragm stop fully open; and the on-axis spacings D5, D10 and D12 (in mm) between the lens groups. Also listed, for the wide-angle end (WIDE) of Prior Art Example 2 are: the back focus Bf (in mm) of the zoom lens; the back focus Bf4b of the rear subgroup of the fourth lens group; the ratio Bf4b/Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R21) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R22) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/FN; and the amount of axial shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 3) 1 mm toward the image so as to adjust the back focus. In the bottom portion of the table are listed the screen size (in mm) for which the zoom lens of Prior Art Example 2 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open.
FIGS. 20(a) and 20(b) show the spherical aberration and the astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 2 when the rear subgroup IVb is positioned at its design position. FIGS. 20(c) and 20(d) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 2 after the rear subgroup IVb has been moved (from the position given in Table 3 above) 1 mm toward the image so as to adjust the back focus. The curves shown in each of FIGS. 20(b) and 20(d) are for the astigmatism in the sagittal (S) and tangential (T) image planes. As is apparent from FIGS. 20(a)-20(d), the zoom lens of Prior Art Example 2 undergoes significant deterioration in spherical aberration and some deterioration in astigmatism as a result of the subgroup IVb being moved 1 mm toward the image so as to adjust the back focus.
The present invention relates to a zoom lens for television cameras, especially to a zoom lens that can be mounted in a camera body such as a CCTV (Closed Circuit television) camera and that has an adjustable back focus.
The object of the present invention is to provide a zoom lens in which a lens component that is moved for adjusting the back focus has a simple and compact structure and the lens performance is maintained even when the back focus is adjusted, as is often necessary where the zoom lens, in use, may be mounted to various T.V. cameras of broadcast quality with the same nominal design, but the mounting flanges from camera to camera vary somewhat due to production tolerances.